1. Technical Field
The present disclosure relates to a method for controlling the humidity level in a cavity of a cooking oven provided with a steam generator.
Typically a domestic cooking oven comprises electrical heating elements which are controlled using a temperature sensor inside the cavity, said sensor closing the loop on a desired cavity temperature and generating, through a control unit, the proper heating element actuation to maintain the desired set temperature.
In a similar way, the steam generator could be included in the same control loop since it contributes as an additional heating element, to control the cavity temperature. Since the steam temperature at atmosphere pressure is around 100° C., this way of controlling the steam generator is valid when the cavity temperature is below 100° C. (pure steam). When working with superheated steam (with cavity temperature above 100° C.), the way of controlling the steam generator is the cavity humidity control. In other words, typically, the steam generator should be actuated in order to provide the right quantity of steam inside the cavity and maintain the desired humidity level inside the cavity itself. The right humidity level inside the cavity is typically dependent on the food category to be cooked. Other parameters that typically drive the humidity level to be maintained inside the cavity are also: recipes, quantity/mass of food, food types, final result of cooking etc.
The control of the humidity level inside the oven cavity requires generally that the steam is generated and controlled in predetermined period/s during the cooking process (and this is disclosed for example by US-A-20060289438, US-A-20060251785 and US-A-20060251784).
The control of the right humidity level inside the cavity of the oven could be achieved by using a humidity sensor located inside the cavity or in contact with air of the cavity through some pipe or chimney. Using this kind of sensor it is possible to close the loop around humidity level actuating the steam generator according to the output of the control unit. In this way, it could be possible to provide the right humidity level inside the cavity. Once the humidity control is in place, it's still necessary to establish the right humidity level, food category by food category or food by food, considering also food weight and cooking performance desired (browning level, juiciness, etc). Some automatic cooking cycles, like the ones described by the above patent applications, could help in this second aspect of the overall picture. Of course, the professional user could manually establish the right humidity level inside the cavity, modifying the humidity set point that the electronic control could provide via a user interface.
The humidity level inside the cavity can be controlled also in a more simply way: the steam generator is actuated in an open loop control system with a different duty cycle depending on the humidity desired. This principle is based on the fact that the humidity level inside the oven cavity is an algebraic sum of the steam injected by the steam generator and the steam sucked or lost by oven chimney or other cavity openings. This second method is clearly simpler than the first presented and of course also cheaper, more robust and easier to be implemented. The drawback of this method is the precision that can be reached: in fact, this second method is affected by the imprecision of all the open-loop control.
Moreover, one of the more important noises disturbing the open loop algorithm is the variation of the voltage value applied to the steam generator. Some documents have already disclosed methods for solving this additional problem, like WO-A-9953252, U.S. Pat. No. 5,990,460 and U.S. Pat. No. 7,081,601. These patent documents disclose the importance of knowing the voltage value when an open-loop algorithm is used in a cooking process, applying a correction of the duty cycle of the heating elements to compensate the voltage variation effect on the power delivered.
In using the above second methodology (open loop control system) to control the humidity level, there is still the need to improve the performance in order to reach control performances similar to a closed loop control system, but without the complexity/cost to install a humidity sensor inside an oven cavity.